1. Technical Field
The present invention relates to a frequency stability measuring apparatus and more specifically to a structure of a frequency stability measuring apparatus that measures a frequency stability of an oscillator under test with a high resolution.
2. Related Art
A method of measuring a phase noise in order to test the frequency stability of a signal source such as an oscillator has already been known. Quadrature detection is commonly used as a method for measuring the phase noise of an oscillator that requires high frequency stability, such as an oscillator used in a communication device or the like. In the quadrature detection, an output of an oscillator under test and an output of a reference oscillator (a voltage controlled oscillator) with less noise than that of the oscillator under test are supplied to a mixer. At this time, a control voltage of a PLL circuit is controlled so that a phase difference between the oscillator under test and the reference oscillator is adjusted to 90 degrees. Then, the sum of a noise component of the oscillator under test and that of the reference oscillator is supplied via a low-pass filter (LPF) to an FFT analyzer to measure the phase noise of the oscillator under test. Thereafter, an analysis by the FFT analyzer, i.e., a phase noise characteristic of the oscillator under test, is displayed on a display screen of a personal computer (PC).
The quadrature detection, however, has a problem in that it takes a long time before starting measurement because it is necessary to adjust the reference oscillator so that the phase difference between the oscillator under test and the reference oscillator will be 90 degrees. Moreover, because it requires use of the FFT analyzer, a spectrum analyzer, or the like, which are costly, the quadrature detection is not suitable as a method for measuring all oscillators mass-produced.
FIG. 5 is a block diagram of a frequency stability testing apparatus disclosed in JP-A-2002-243778, which has addressed such a problem of the known quadrature detection. This frequency stability testing apparatus 21 includes an oscillator 22 under test, a plurality of oscillators 23 having different oscillation frequencies, a power supply 24 for driving these oscillators 22 and 23, a switching circuit 25, a mixer 26, a filter 27, a counter 28, and a microcomputer 29. It is suggested that this exemplary related art provides an apparatus and method for testing the frequency stability that are capable of testing the frequency stability of a signal source, such as an oscillator, easily and in a short time.
JP-A-2002-243778 is an example of related art.
This exemplary related art measures a difference frequency between the two oscillators by inputting, to the counter, a signal obtained by passing an output of the mixer through the filter. Measuring this signal in a rapid manner requires use of a reciprocal counter, but it is known that the reciprocal counter causes increase of measurement error depending on a slew rate of an input signal. The technique of the exemplary related art decreases the slew rate to an extreme extent because of the filter, and therefore this technique has a problem in that it is difficult to achieve faster and more precise measurement.
Moreover, in the case where a counter gate time is set to about 10 ms, a frequency resolution of about 0.1 mHz is required to measure the stability of a crystal oscillator with a sufficient resolution. However, general reciprocal counters achieve a resolution of about 10 mHz only (see FIG. 4). Therefore, in the case of measuring objects with high stability such as crystal oscillators, there are problems of an insufficient resolution, a long measurement time, etc.